The present invention pertains to linearly polarized notch (i.e., slotline) antennas that are tapered outward toward the open end. As is known, an open-ended slot or notch radiator is a relatively broadband element especially when flared as a broadband transition to free space. It has important advantages which are desirable, such as being light in weight, cheaply manufactured with printed circuit board techniques that are capable of accurate replication from unit to unit.
Tapered notch antennas excited by a microstrip feedline are known in the art. Such a prior art antenna is shown in FIG. 1. There is shown a planar surface 101 such as a circuit board with a front side 103 and a back side 105. The front side 103 has a metallized surface 107 with a tapered notched area 111 etched away to expose a dielectric substrate 109. This area extends to the edge finalized as dimension A. The back side 105 comprises the dielectric substrate 109 with a metallized strip 113 affixed thereon. The metallized surface 107 forms a ground plane for the microstrip feed line 113.
As is known, the signal to be transmitted is applied to the strip 113 and coupled to the tapered notch 111 by means of the cross-over junction 115. The length L.sub.1 of the open circuit stub 117 of the strip 113, and the length L.sub.2 of the short circuited stub of the notch 111 are adjusted for optimum coupling at the junction 115. A notch antenna begins to radiate when the width of the notch as manifested by the taper becomes excessively wide. It is known that if the guide wavelength in the notch exceeds about 0.4 free space wavelength, then radiation results. The radiation may be controlled by the taper as a travelling wave outward toward the flared open end A. The dielectric helps confine the fields to within the region of the notch. At that point, nearly matched impedance conditions exist and launch of the field occurs with the E- and H-field components and the maximum radiation direction P as indicated. The wave polarization is parallel with the plane of the notch and the attendant taper. A phase center exists essentially at the center of the end of the flare A, and reciprocity holds for the system.
The radiation pattern in the E-plane has maximum directivity in the direction of P determined, in part, by the elecrical dimension of A. The H-plane radiation pattern has a very broad cardioid shape with a deep null in the direction of the shorted end of the notch and the maximum at the taper end in the direction of P.
One problem with the prior art arrangement, as in FIG. 1, is that it has low directivity in the H-plane. It is desirable, therefore, to provide an improved tapered notch antenna.